This invention relates to rotatable assemblies for dynamoelectric machines, such as electronically commutated motors. More particularly, it relates to a means for reducing shedding of particles from the permanently magnetizable materials which are typically employed to form the magnet elements for such machines.
In conventional designs of rotatable assemblies for dynamoelectric machines, such as, for example, electronically commutated motors, permanent magnet elements are fastened to the outer surface of a steel drum which is attached to a rotatable shaft. In such designs, the permanent magnet elements form the outer surface of the rotor assembly. The permanent magnet elements typically used in electronically commutated motor rotors are formed from permanently magnetizable material which has been made by powder metallurgical techniques. These materials are brittle and have a tendency to release microscopic particles or chips from their surfaces, especially during rotation of the rotor assembly. Some of these released particles may migrate through the stator-rotor air gap and settle on the inner surface of the stator, thereby affecting the stator-rotor gap and degrading motor performance.
Various schemes have been employed to retain magnet material elements, such as permanent magnets or the like, against displacement from their proper positions in a rotatable assembly for dynamoelectric machines. It would appear that some of these schemes would also serve, at least in part, to reduce migration of magnet material particles into the stator-rotor gap. One such scheme is disclosed in U.S. Pat. Nos. 4,242,610, issued Dec. 30, 1980 to F. B. McCarty et al., and 3,531,670, issued Sept. 29, 1970 to D. C. Loudon. Those patents appear to describe heat shrinking a metal or metal alloy sleeve or band about the rotor core into displacement-preventing engagement with a set of magnet material elements arranged or otherwise seated in assembly positions about the circumference of the rotor core. While such an arrangement appears to at least partially reduce migration of magnet material particles from the outer surfaces of the magnet elements, this prior art arrangement requires undesirable heating during assembly, is too bulky and cumbersome for some applications, and is relatively expensive. To provide the metallic bands disclosed in those patents with adequate strength to hold the magnet elements in position, a relatively thick material must be used. Furthermore, even if the magnet elements are fastened to the rotor core by a supplementary means such as adhesive bonding, it is still difficult to provide such bands with adequate strength using a thin material. In addition to the hoop stresses in the metallic band generated during rotation of the rotor assembly by centrifugal forces acting on the band's own mass, radial expansion of the rotor assembly during rotation also imposes stresses on the band. The amount of radial expansion of the rotor assembly is expected to be greater than that of the metallic band for several reasons. First, during rotation the flux ring exhibits an incremental expansion due to centrifugal loading by the attached magnet elements. Also, if the magnet elements are adhesively bonded to the flux ring, the magnet elements tend to move radially at a different expansion rate than that of the metallic band, due to the low elastic modulus of the adhesive. Additionally, if the rotor core is made of plastic, the rotor core also tends to radially expand at a different rate than the metallic band, because of the core's lower elastic modulus. To withstand the stresses imposed by these mechanisms, the metallic bands disclosed by U.S. Pat. Nos. 4,242,610 and 3,531,670 appear to require relatively high mechanical strength. U.S. Pat. No. 3,221,194, issued Nov. 30, 1965 to A. B. Blackburn, also appears to disclose a scheme which would tend to reduce migration of magnet material particles into the stator-rotor gap. That patent describes dipping a rotor core with permanent magnet material elements arranged thereon into a plastic bath. When cured, the plastic forms an encapsulating layer over both the rotor core and the magnet elements, securing them together. However, for similar reasons as discussed above, such a plastic layer would also be subjected to both hoop stresses and stresses imposed by radial expansion of the rotor assembly during rotation.
Accordingly, it is an object of the present invention to provide a magnet assembly having means for reducing release of magnet material particles from the surface of the magnet element.
It is a further object of the present invention to provide a rotatable magnet assembly in which the stresses imposed on the means employed for reducing release of magnet material particles from the magnet elements, caused by radial expansion of the assembly during rotation, are reduced.
It is another object of the present invention to provide a rotatable assembly for dynamoelectric machines in which migration of magnet material particles into the stator-rotor gap is reduced.